The metals like Al, Ti, Mg and their alloys are commercially and widely used in the engineering industries like automobile, aerospace, textile, petrochemical and crockery in the form of rods, bars, tubes, sheets, pipes, channels, sections, pulleys, cylinders, pistons etc. Apart from the specific promising properties and commercial availability that these materials have, the main reason for using these materials is its high strength to weight ratio. However, there exists a limitation to use these materials beyond a certain point, the limitation arises from the fact that these materials exhibit poor resistance to wear and tear, chemical attack and heat.
Traditionally, anodizing is employed to obtain coatings on Al-alloys. But the resultant coatings are found to be porous, weekly adherent to the substrate, thereby can not provide high level protection against wear and tear and corrosion. More over, coating deposition rates achieved are also low in the anodizing process.
Thermal spraying techniques like plasma spraying, high velocity oxy fuel spraying, detonation spraying are well developed and widely used by the engineering industry to produce large varieties of metallic, oxide, carbide and nitride based ceramic coatings. These coatings are essentially employed to combat various forms of wear and tear and corrosion thereby to enhance the servie life of the components made of different metals and alloys. However, thermal spray techniques demand a high degree of pre coating and post coating operations which are often cost inductive. Size, shape and complexity in geometry of the engineering components do restrict the applicability of the thermal spray techniques. Moreover, these techniques demand high quality as well as costly powders such as Alumina, Alumina-Titania, Tungsten Carbide-Cobalt, Chromium Carbide-Nickel Chrome prepared by specially developed manufacturing routes such as sol-gel, atomization, fusing, sintering & crushing, chemical reduction and blending. Deposition efficiency of these powders is always much less than 100% thus requiring a special means of unused powder separation from the coating chamber. Since these coating techniques employ spraying of heated powder particles on the relatively cold surfaces, often results in poor metallurgical bonding between the substrate and the coating. These coatings are often characterized by inherent porosity, micro cracks and higher levels of residual stresses which in turn leads to the failure of the coatings in the case of critical applications.
To overcome the above mentioned difficulties and limitations and the present day need for coatings exhibiting improved tribological, electrical, thermal and chemical properties and having higher density and excellent wear resistance research work in the area of developing an improved micro arc oxidation process has gained importance globally.
There exist a good number of patents and publications which deal with the micro arc oxidation processes of aluminum and its alloys. Some relevant literature on prior art micro are processes are referred to below.
According to U.S. Pat. No. 6,197,178, a three phase pure sinusoidal potential of 480V AC electrical power is supplied to aluminium alloy bodies and current densities between 20 and 70 A/dm2 is applied. During the process, current density is maintained by moving the bodies relative to each other. An electrolyte with KOH, Na2SiO3 and Na2O.Al2O3.3H2O in the proportion of 2 gram per liter of de-ionized water is used. Temperature of the electrolytic bath is maintained between 25 degree C. and 80 degree C. The coating thickness achieved is reported to be in the range of 100 to 160 microns for a 30 minute processing time on cylindrical samples.
Although the resultant coatings were identified to have strong adherence with the substrate no information is available with respect to the density and uniformity of the coatings achieved. Coating density is very important parameter in deciding the wear resistance of the resulting coatings.
In the invention cited above, the inventors used a pure sinusoidal voltage wave form without any waveform modification, while a sharply peaked-waveform makes a major contribution in providing a dense and hard coating. This is why the coatings obtained through the above mentioned process exhibit lower hardness ie. 1200-1400 kg/mm2.
U.S. Pat. No. 5,616,229 granted to Samsonov et al. discloses a method of forming a ceramic coating on valve metals. This method comprises application of at least 700V alternating current across the parts to be coated. Waveform modification is achieved through a capacitor bank connected in series between high voltage source and the metallic body to be coated. Waveform of the electric current rises from zero to its maximum hight and falls to below 40%. of its maximum height with in less than a quarter of a full alternating cycle.
Electrolyte used in the above cited, process contains 0.5 grams/liter NaOH, 0.5-2 grams/liter KOH. In addition, electrolyte also contains sodium tetra silicate for which there is no claim on the exact amount to be added. During the process, the electrolyte composition is changed by adding oxy acid salt of an alkali metal in the concentration range of 2 to 200 grams per liter of solution. This process has been demonstrated by coating an aluminium alloy known as Duralumin by employing 3 different electrolytic baths.
However, in the processes explained above the applicants did not maintain any particular ratio between the alkali and the metal silicate.
In the micro are oxidation process, alkali is actually responsible for dissolving the coating where as the metal silicate is responsible for coating built up through poly condensation of silicate anions. Too high silicate concentration in the electrolyte causes higher coating built up especially at the sample edges rather than at the other portions of the sample thus resulting in a non-uniform coating. Hence there is a need to maintain a certain degree of proportion between the alkali and metal silicate in order to end up with a uniform and dense coatings.
Further, the process disclosed in the U.S. Pat. No. 5,616,229 has been claimed to have an average deposition rate of 2.5 micron per minute. However, the thickness of fully melted inner layer is only 65 microns out of a total coating thickness of 100 microns. This indicates that this process can produce coatings comprising only 65% initial dense layer and remaining 35% external layer is porous with 4-6 no. of pores per sq.cm. area and an average pore diameter of 8-11 microns.
To make these coatings suitable for wear resistant applications, the external porous layer of sufficient thick needs to be completely removed by machining or grinding. Apart from the fact that these machining or grinding operations are costly, machining/grinding of coated parts of complex, non-symmetric shapes is extremely difficult and demands high degree of automated machinery and higher skill levels also. This effictively increases the cost of the coating per unit volume.
The prior art processes of micro anrc oxidation processes through yielded thick, adherent coatings with higher coating deposition rates but failed to produce dense and uniform layers which are essentially required to impart high hardness, higher wear resistance against abrasion, sliding and erosion wear modes as well as with relatively better surface finish. Also, coatings with higher fraction of inherent porosity will not give satisfactory corrosion resistance and dielectric properties.
Moreover, in the prior art, the process employed for coating metallic bodies has been discussed in detail, but not much has been disclosed about the apparatus used for carrying out the coating process.
According to the invention disclosed in U.S. Pat. No. 6,197,178, the apparatus employed for obtaining the coating consists of a chemically inert coating tank disposed with in an outer tank. The outer tank contains heat exchange fluid. Electrolyte from the inner tank is circulated through the heat exchanger disposed in the outer tank itself. To remove heat from the heat exchange fluid, heat exchange fluid is withdrawn from the outer tank with the help of a pump and then passed through a forced air cooled heat exchanger. The operation of the exchangers was controlled automatically so as to maintain the desired temperature within the electrolyte bath. However, there exists a serious drawback with this kind of setup. When a component of larger size than that of the inner coating tank is to be coated, the dimensions of the inner tank are to be increased which in turn may demand for changing the outer tank dimensions as well. This makes the process more cost inductive.